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Thermal and fluid dynamic performance of pin fin heat transfer surfaces [Elektronische Ressource] / vorgelegt von Naser Sahiti

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173 pages
Thermal and Fluid Dynamic Performance of Pin Fin Heat Transfer Surfaces Der Technischen Fakultät der Universität Erlangen-Nürnberg zur Erlangung des Grades DOKTOR-INGENIEUR vorgelegt von Naser Sahiti Erlangen, 2006 Als Dissertation genehmigt von der Technischen Fakultät der Universität Erlangen-Nürnberg Tag der Einreichung: 17. Oktober 2005 Tag der Promotion: 27. Januar 2006 Dekan: Prof. Dr.-Ing. A. Leipertz Berichterstatter: Prof. Dr. Dr. h. c. F. Durst Prof. Dr.-Ing. W. Arlt Ass. Prof. Dr. A. Dewan Acknowledgments During my work on the present thesis I have been fortunate to interact with many people who helped me in one way or another to complete this project. It gives me great pleasure to have the opportunity to address my special thanks to them. First of all I express my deepest thanks to my supervisor, Prof. F. Durst, who gave me the op-portunity to finish the thesis in his institute. His creative ideas, encouragement and valuable criticism have profoundly contributed to the completion of the present thesis. I remain greatly indebted to him for his permanent guidance and his continuing disposition in discussing various aspects of the work. I express special thanks to Prof. W. Arlt and Prof. A. Dewan for their willingness to write evaluations of my thesis. Thanks are due to Prof. A.
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Thermal and Fluid Dynamic Performance
of Pin Fin Heat Transfer Surfaces


Der Technischen Fakultät der
Universität Erlangen-Nürnberg
zur Erlangung des Grades


DOKTOR-INGENIEUR



vorgelegt von

Naser Sahiti

Erlangen, 2006





















Als Dissertation genehmigt von
der Technischen Fakultät der
Universität Erlangen-Nürnberg



Tag der Einreichung: 17. Oktober 2005
Tag der Promotion: 27. Januar 2006

Dekan: Prof. Dr.-Ing. A. Leipertz
Berichterstatter: Prof. Dr. Dr. h. c. F. Durst
Prof. Dr.-Ing. W. Arlt
Ass. Prof. Dr. A. Dewan



Acknowledgments
During my work on the present thesis I have been fortunate to interact with many people who
helped me in one way or another to complete this project. It gives me great pleasure to have the
opportunity to address my special thanks to them.
First of all I express my deepest thanks to my supervisor, Prof. F. Durst, who gave me the op-
portunity to finish the thesis in his institute. His creative ideas, encouragement and valuable
criticism have profoundly contributed to the completion of the present thesis. I remain greatly
indebted to him for his permanent guidance and his continuing disposition in discussing various
aspects of the work.
I express special thanks to Prof. W. Arlt and Prof. A. Dewan for their willingness to write
evaluations of my thesis. Thanks are due to Prof. A. Dewan also for excellent cooperation dur-
ing his guest visit at the Institute of Fluid Mechanics in Erlangen. I also thank Prof. R. Singer
and Prof. P. Brunn for their willingness to interact in the thesis committee.
I am grateful to the staff of the administration office, Dr. B. Mohr, M. Grim, M. Hill and N.
Zink, and to the staff in the secretarial office I. Paulus, J. Grasser and I. Knopf, for all their valu-
able help in numerous projects during my thesis work. Further, I greatly appreciate the help and
suggestions of the staff of the mechanical workshop, J. Heubeck, J. Svjeda, H. Hedwig and J.
Sippel. Thanks are also due to H. Weber and R .Zech for their competent support in completing
the electronic aspects of the experiments.
I address particular thanks to my friends A. Peugnet, A. Lemouedda, D. Stojkovi ć, M. Pascu and
S. Mayer for their generous assistance in various phases of the present thesis. I am grateful to
Prof. E. Franz, Prof. S. Chakraborty, Dr. G. Sieber and V. Kumar for their insightful reviews of
the some thesis chapters. Further thanks are due to Prof. M. Breuer and U. Fröhner for their help
and valuable suggestions, and further to my colleges and friends Ö. Ertunç, P. Epple, D. Kosso-
lapov, B. Ünsal, F. Avdi ć, A. Basara, E. Zanoun, O. Saleh, C. Köksoy, I. Paramasivam, K.
Haddad, V. Stamatov, H. Lienhart, B. Frohnapfel, M. Kretschmar and Y. Abu-Sharekh for their
help, suggestions and fruitful discussions and for the good times we spent together.
I greatly acknowledge the financial support from German Academic Exchange Service (DAAD)
in Bonn and the Institute of Fluid Mechanics in Erlangen during my thesis work.
Finally I thank my parents, who instilled in me the value of the education, my wife Hazbije, for
her support, patience and being on my side all the time, and our son Erjon, who was so happy
whenever he saw me coming back from the Institute.


Abstract
The research work summarized in this thesis presents a combined analytical, experimental and
numerical investigation of various aspects of single-phase convective heat transfer enhancement
by the use of pin fins is presented. After a brief review of the basic methods used to enhance the
heat transfer by simultaneous increase of heat transfer surface area as well as the heat transfer
coefficient, a simple analytical method to assess the heat transfer enhancement is presented. The
method is demonstrated on pin fins as elements for the heat transfer enhancement, but it can in
principle be applied also to other fin forms. In order to check the applicability of the analytical
method, experimental investigations of a double-pipe pin fin heat exchanger were carried out.
The order of the magnitude of heat transfer enhancement obtained experimentally was similar to
that obtained analytically. The heat transfer and pressure drop results for the pin fin heat ex-
changer were compared with the results for a smooth-pipe heat exchanger. It was found that by a
direct comparison of Nu and Eu, no conclusion regarding the relative performances could be
made. This is because the dimensionless variables are introduced for the scaling of heat transfer
and pressure drop results from laboratory to large scale but not for the performance comparison.
Therefore a literature survey of the performance comparison methods used in the past was also
performed. It was found that all proposed methods in the literature offer only an approximate
comparison of the performance of heat transfer surfaces. For new developments in heat transfer
enhancement methods, it was considered that such methods would fail to predict the perform-
ance of new heat transfer surfaces. Hence in the present thesis a more consistent comparison
method of the performance of heat transfer surfaces is proposed and its applicability demon-
strated.
The new comparison method compares directly the heat transfer rate with the required pumping
power of heat transfer surfaces under comparison. The heat exchanger volume as another impor-
tant parameter for the heat exchanger design is considered by plotting the heat transfer rate ver-
sus power input normalized to the heat exchanger volume. The heat exchanger performance plot
obtained in this way allows the comparison of newly tested heat transfer surfaces and also of
heat transfer surfaces with available heat transfer and pressure drop data. Depending on the con-
straints used during the comparison, the heat exchanger performance plot allows the comparison
of the performance of entire heat exchangers in their actual state or of the heat transfer surfaces
only.
The newly developed performance comparison method allowed a detailed numerical study of
the influence of pin cross-section on the performance of pin fin arrays used in the electronics
industry. In order to cover a wide range of influencing parameters in the performance compari-
son, two geometrical comparison criteria for six different cross-sections with both in-line and
staggered arrangements were selected.
One of the intentions of the present work was the derivation of the basic heat transfer and pres-
sure drop data for pin fins which might be applied as an alternative to common interrupted fin
forms (strip or louvered fins) in flat-tube heat exchangers used in the air conditioning and auto-
mobile industries. Therefore, a comprehensive parametric study of small-diameter pins with
high a population density was carried out. For the presentation of the data in terms of dimen-ii Abstract


sionless variables (Nu and Eu) as functions of all important parameters, a multiple regression
analysis was performed. The overall performance was compared based on the heat exchanger
performance plot.
However, it was found that for a proper comparison of flat-tube and pin fin heat exchangers,
some correction of pin the length is required. This is because the entire heat exchanger perform-
ance is influenced not only by pin length but also by the blockage factor of the heat exchanger
frontal area by the walls of the tube carrying liquid on the other side of the heat exchanger. The
analysis of the performance parameters of the pins with such a corrected pin length and the per-
formance comparison based on heat exchanger performance plot resulted in a similar optimal
pin length.
In the last part of the thesis, comparison of the overall performance of a common flat-tube and
louvered fin heat exchanger with a model of a flat-tube and pin fin heat exchanger is made. For
comparison purposes, a louvered fin heat exchanger model was experimentally tested. The pin
fin heat exchanger was numerically simulated for the same thermal and fluid dynamic boundary
conditions. From a performance comparison based on the heat exchanger performance plot, it
was found that the pin fin heat exchanger is able to perform the same way as a louvered fin heat
exchanger but with 22% less volume.


Nomenclature
A Area
b Plate distance by plate-fin heat exchangers
Bi Biot number
C Heat capacity rate
c Specific heat at constant pressure p
C Sutherland constant s
d Diameter
D Diameter of the outer pipe by double pipe heat exchangers
e Energy normalized to area ore volume
E Energy
Eu Euler number
f Friction factor
F Dimensionless pumping power factor
h Heat transfer coefficient, Height
j Colburn factor
J Dimensionless heat transfer factor
k Thermal conductivity
l Length
m Fin parameter
m& Mass flow rate
N Number of pin (tube) rows
NTU Number of heat transfer units
Nu Nusselt number
p Pressure
P Perimeter
Pr Prandtl number
&Q Heat flux
Re Reynolds number
S Streamwise spacing L
s Minimum distance between pins m
St Stanton number iv Nomenclature


S Transverse spacing T
T Temperatue
U Overall heat transfer coefficient
u,v,w Cartesian velocity comonents
V Volume
&V Volume flow rate
Greek Letters
β Heat exchanger compactness
δ Characteristic fin dimension (in Biot number)
∆ Difference
ε Effectiveness, Heat exchanger efficiency
η Efficiency
κ Roughness
µ Dynamic viscosity
ν Kinematic
ρ Density
σ Ratio of the free flow area to the frontal area
τ Momentum transport term
ϕ Coverage ratio
Subscripts
a Air
ach Area change
acc Acceleration
ain Inlet air temperature
aout Outlet air temperature
b Bare surface
bf Base of the fin; Performance figure
bp Base of the pin
c Cross section; Cupper; Core
ch Channel
c,in Inlet temperature of cold fluid
cp Corrected pin length Nomenclature v


e Enhancement
f Fin
fl Flow length
fpw First pin wall
fr Free frontal area
h Hydraulic
h,in Inlet temperature of hot fluid
i Inner diameter
in Inlet
lm Logarithmic mean
lpw Last pin wall
m Mean
mf Mean fluid temperature
min Minimal
mn Micro-manometer
n Soland parameters, Nozzle
o Primary surface; outer diameter
out Outlet
p Pin
prrows
s Solid
t Tip of the fin; Total area; Total surface; Profile thickness
uf Unfinned part of the base plate
up Unpinned part of the base plate
v Volume
w Water; Wall
win Inlet water temperature
wout Outlet water temperature
∞ Free fluid stream


Contents
Acknowledgments ................................................................................................................................ i
Abstract................................................................................................................................................. i
Nomenclature......................................................................................................................................iii
Contents....................... vi
Chapter 1.............................................................................................................................................. 1
Introduction....................... 1
1.1 State of the Art of Heat Transfer Enhancement Techniques ..................................................... 1
1.2 Aim of the Work ........................................................................................................................ 4
1.3 Organization of the Thesis......................................................................................................... 5
Chapter 2........................ 7
Preliminary Considerations on Heat Transfer Enhancement Methods................................................ 7
2.1 Characteristics of Some Effective Heat Transfer Surfaces........................................................ 7
2.2 Fin Performance Parameters... 12
2.3 Order of Magnitude Considerations for Heat Transfer Enhancements ................................... 17
Chapter 3............................................................................................................................................ 20
Experimental Investigation of a Counter-flow Pin Fin Heat Exchanger ........................................... 20
3.1 Thermal and Fluid Dynamic Characteristics of the Flow through Pin Fin Arrays.................. 20
3.2 Literature Review of Heat Transfer from Pin Fins .................................................................. 21
3.3 Heat Exchanger Test Rig ......................................................................................................... 24
3.4 Experimental Procedure and Data Reduction.......................................................................... 26
3.5 Uncertainty Analysis................................................................................................................ 29
3.6 Discussion of the Results......................................................................................................... 30
Chapter 4............................................................................................................................................ 33
Selection Strategy of Elements for Heat Transfer Enhancement....................................................... 33
4.1 Introduction Remarks....... 33
4.2 Review of Comparison Methods for Heat Exchanger Selection ............................................. 34
4.3 Approximate Comparison of Pin Fin Heat Exchanger versus Smooth Pipe Heat Exchanger. 38
4.3.1 Heat Transfer Coefficient as Performance Variable......................................................... 38
4.3.2 Number of Heat Transfer Units as Performance Variable................................................ 41
4.4 Consistent Comparison of Pin Fin Heat Exchanger versus Sm..... 42
4.5 Consistent Comparison of Heat Exchanger Surfaces with Known Characteristics................. 46
4.6 Discussion of Results and Final Remarks................................................................................ 49
Chapter 5............................................................................................................................................ 51
Numerical Investigations of the Influence of Pin Fin Cross-Section on Heat Transfer and
Pressure Drop..................................................................................................................................... 51
5.1 Introduction Notes ................................................................................................................... 51
5.2 Criteria Applied for Comparison ............................................................................................. 51
5.3 Geometric Characteristics and Pin Fin Arrangement .............................................................. 53

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